Xprops cleanups

While working on the mutter patches for Xwayland randr resolution change emulation, I noticed thet the xprops.c code uses a mix of free, g_free and XFree, and often uses the wrong function in relation to how the memory was allocated.

This series fixes this, converting all malloc calls to g_malloc calls and fixing all callers (including those of methods which already used g_malloc) to use g_free consistently.