add missing resize for framless window on linux

[lyimmi]

This PR fixes #1917.

I tried to follow the style of the windows implementation, but there are some differences:

1. There is no throttling of the resize event. (It looks like GTK handles it well.)
2. Since there is no navigationCompleted() on linux, I wired the first f.ExecJS("window.wails.flags.enableResize = true;") to the runtime:ready event.
3. Created a new global variable mouseButton like the x,y coordinates for gtk_window_begin_resize_drag

Tested it on linux/amd64 & linux/arm, however I'm not too advanced on GTK/GDK/C so a cgo wizard should take a good look at it. :)

[leaanthony]

Thanks for this awesome PR!

[lic-8]

@lyimmi Excellent work thank you !

@leaanthony When will this be in a realease version?

[leaanthony]

The weekend hopefully 👍