overrides/Gio: Add GObject/promise-based D-Bus convenience

Add a new Gio.DBusProxy.createProxySubclass() method that is used
the same way as Gio.DBusProxy.makeProxyWrapper(), but:

 - creates a real Gio.DBusProxy subclass with all D-Bus signals
   and properties turned into GObject equivalents (no more
   connectSignal() and friends)

 - have all methods that proxy a remote method return a promise

 - have a static new() method similar to Gio.DBusProxy.new(),
   except that it returns a promise
25 jobs for wip/dbus-promise in 17 minutes and 35 seconds (queued for 29 minutes and 11 seconds)
latest
Status Job ID Name Coverage
  Source Check
passed #73160
cppcheck

00:00:35

passed #73161
cpplint

00:00:55

passed #73162
eslint

00:00:37

passed #73159
fedora

00:02:27

 
  Test
passed #73165
no_graphics

00:02:05

passed #73166
no_profiler

00:02:10

passed #73167
no_readline

00:02:10

passed #73163
sanitizer_gcc

00:04:05

passed #73164
with_logging

00:02:26

 
  Manual
manual #73181
allowed to fail manual
armv8
manual #73168
allowed to fail manual
codequality
manual #73169
allowed to fail manual
code_statistics
manual #73183
allowed to fail manual
coverage
manual #73171
allowed to fail manual
fedora_gcc
manual #73177
allowed to fail manual
flatpak
manual #73172
allowed to fail manual
installed_tests
manual #73175
allowed to fail manual
lts
manual #73182
allowed to fail manual
ppc64le
manual #73170
allowed to fail manual
sanitizer_clang
manual #73174
allowed to fail manual
ubuntu_clang
manual #73173
allowed to fail manual
ubuntu_gcc
manual #73176
allowed to fail manual
valgrind
manual #73180
allowed to fail manual
zeal_11
manual #73178
allowed to fail manual
zeal_2
manual #73179
allowed to fail manual
zeal_4