Replace the manual browser opening code with gtk_show_uri_on_window

Remove the browser preference page.

replace the manual open uri code with gtk_show_uri_on_window

Testing Done:
Compiles, preference window functions properly, however talkatu doesn't handle links correctly so I couldn't test from messages or `help -> about`, but `help -> online help` worked just fine.

Reviewed at https://reviews.imfreedom.org/r/59/