main: Add a few missing header include paths

https://bugzilla.gnome.org/show_bug.cgi?id=702631

Conflicts:
	src/Makefile.am
main: Add helper function to set RTL icons

https://bugzilla.gnome.org/show_bug.cgi?id=702631
main: Set button arrow icons according to locale's text direction

media-playback-start-symbolic in LTR,
media-playback-start-rtl-symbolic in RTL.

media-seek-forward-symbolic in LTR,
media-seek-forward-rtl-symbolic in RTL.

media-seek-backward-symbolic in LTR,
media-seek-backward-rtl-symbolic in RTL.

media-skip-forward-symbolic in LTR,
media-skip-forward-rtl-symbolic in RTL.

media-skip-backward-symbolic in LTR,
media-skip-backward-rtl-symbolic in RTL.

https://bugzilla.gnome.org/show_bug.cgi?id=702631
browser-plugin: Set button arrow icons according to locale's text direction

media-playback-start-symbolic in LTR,
media-playback-start-rtl-symbolic in RTL.

https://bugzilla.gnome.org/show_bug.cgi?id=702631
browser-plugin: Don't disable deprecated functions

We still rely on the stock API.

https://bugzilla.gnome.org/show_bug.cgi?id=702631