object: Use std::vector to hold Objects GClosure's

We used a std::forward list to hold closure pointers in Object's to avoid adding a vector when not needed, but these structures are terribly slow as already pointed out in [1], and demonstrated even way poorer performances when testing signal connections (see [2], #485 (closed) and !758 (merged)), so get rid of them and just use vectors everywhere which can boost performances up to 95% in our tests.

This implies some more memory usage for each object, as we can't use a std::vector without the size member since C++11, but I think it's still better, given that size of std::forward_list is still bigger when with only two closures/vfuncs added (as each element requires two pointers).

Fixes: #485 (closed)

[1] https://github.com/3v1n0/stl-containers-benchmarks [2] 3v1n0/gjs$3608

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It's quite crazy, but just switching to a std::vector we easily go from:

Gjs-Console-Message: 02:51:56.398: connectToManyDifferentSignals(nConnections=15000, nHandlersEach=1): 1293.256 ms
Gjs-Console-Message: 02:51:56.450: emitAllSignals(nConnections=15000): 49.005 ms
Gjs-Console-Message: 02:51:56.540: emitAllSignalsRandomly(nConnections=15000): 73.072 ms
Gjs-Console-Message: 02:51:57.892: disconnectAllSignalsRandomly(nConnections=15000): 1342.209 ms
--------------------
Gjs-Console-Message: 02:51:58.061: connectToManyDifferentSignals(nConnections=5000, nHandlersEach=1): 145.047 ms
Gjs-Console-Message: 02:51:58.207: disconnectAllSignalsRepeatedly(nConnections=5000): 145.534 ms
--------------------
Gjs-Console-Message: 02:51:58.338: connectToManyDifferentSignals(nConnections=5000, nHandlersEach=1): 129.976 ms
Gjs-Console-Message: 02:51:58.525: disconnectAllSignalsRandomly(nConnections=5000): 183.376 ms
--------------------
Gjs-Console-Message: 02:52:15.683: connectToManyDifferentSignals(nConnections=500, nHandlersEach=100): 17155.549 ms
Gjs-Console-Message: 02:52:15.719: emitAllSignals(nConnections=500): 33.461 ms
Gjs-Console-Message: 02:52:15.751: emitAllSignalsRandomly(nConnections=500): 31.143 ms
Gjs-Console-Message: 02:52:44.384: disconnectAllSignalsRandomly(nConnections=50000): 28594.481 ms

to:

Gjs-Console-Message: 02:55:58.950: connectToManyDifferentSignals(nConnections=15000, nHandlers=1): 67.436 ms
Gjs-Console-Message: 02:55:58.996: emitAllSignals(nConnections=15000): 42.633 ms
Gjs-Console-Message: 02:55:59.073: emitAllSignalsRandomly(nConnections=15000): 62.663 ms
Gjs-Console-Message: 02:55:59.193: disconnectAllSignalsRandomly(nConnections=15000): 107.373 ms
--------------------
Gjs-Console-Message: 02:55:59.232: connectToManyDifferentSignals(nConnections=5000, nHandlers=1): 17.014 ms
Gjs-Console-Message: 02:55:59.253: disconnectAllSignalsRepeatedly(nConnections=5000): 20.323 ms
--------------------
Gjs-Console-Message: 02:55:59.268: connectToManyDifferentSignals(nConnections=5000, nHandlers=1): 14.406 ms
Gjs-Console-Message: 02:55:59.295: disconnectAllSignalsRandomly(nConnections=5000): 22.310 ms
--------------------
Gjs-Console-Message: 02:55:59.801: connectToManyDifferentSignals(nConnections=500, nHandlers=100): 503.625 ms
Gjs-Console-Message: 02:55:59.832: emitAllSignals(nConnections=500): 29.394 ms
Gjs-Console-Message: 02:55:59.863: emitAllSignalsRandomly(nConnections=500): 29.829 ms
Gjs-Console-Message: 02:56:00.622: disconnectAllSignalsRandomly(nConnections=50000): 720.122 ms

Which means:

	std::forward_list	std::vector
connectToManyDifferentSignals(15000, 1)	1293,256	67,436
emitAllSignals(15000)	49,005	42,633
emitAllSignalsRandomly(15000)	73,072	62,663
disconnectAllSignalsRandomly(15000)	1342,209	107,373
connectToManyDifferentSignals(5000, 1)	145,047	17,014
disconnectAllSignalsRepeatedly(5000)	145,534	20,323
connectToManyDifferentSignals(5000, 1)	129,976	14,406
disconnectAllSignalsRandomly(5000)	183,376	22,31
connectToManyDifferentSignals(500, 100)	17155,549	503,625
emitAllSignals(500)	33,461	29,394
emitAllSignalsRandomly(500)	31,143	29,829
disconnectAllSignalsRandomly(500)	28594,481	720,122