Revert "Don't use deprecated GLib threading API."

This reverts commit 13f1c75845c8a55a95d2c5426dbf343e3828bc78.

Has an unrelated change included so we need to split this.
Don't use deprecated GLib threading API.
Define EomExifData type using G_DEFINE_BOXED_TYPE macro

Based on eog commit 3cc45bdc35fbfd14627d266c0344f677d2037dcb
From Felix Riemann <friemann@gnome.org>
Handle EomScrollView's instance initialization in the right function

Moves most of the other stuff done in eom_scroll_view_new
over to eom_scroll_view_init.

Based on eog commit e7243a9ddad0f31f8759aadbd92ad551781b8480
From Felix Riemann <friemann@gnome.org>
Bind zoom multiplier to GSettings instead of setting it by hand

Based on eog commit 0cadaebd5c7561991eceaf3ed9af32f50e63b1e9
From Felix Riemann <friemann@gnome.org>
Switch scrollwheel zoom setting with GSettings

Based on eog commit 8c7cf9f8938673eda2843be650b558084b44303e
From Felix Riemann <friemann@gnome.org>
Have the ThumbNav's scrollbuttons bound to the GSettings key

Based on eog commit 0a898703a9a19589e049a73d21c91898b0dbc51e
From Felix Riemann <friemann@gnome.org>
Rework gsettings code to be more like eog