Use gstreamer 1.0 on meique based builds.

Reviewed by: Trust me.
Rename PlatformClient to BrowserPlatform.

PlatformClient inherits Nix::Platform, so the new name make more sense.

Reviewed by: Luciano Wolf
Add gstreamer-pbutils to make Drowser compile and link again with meique.
Remove memory leak and code bloat from Drowser platform initialization code.

Reviewed by: Tullio Lucena
Allowing new repaints only when the current painting is finished

Currently, the callUpdateDisplay and scheduleUpdateDisplay calls happens
on the same thread, all triggered by the same glib mainloop, so it's
working as expected. This patch just moves the flag assigment to the end
of the callUpdateDisplay function in order to improve the code
readability.

Reviewed by: Hugo Parente Lima