Mixing ForkAwesome icons too much with the regular icon theme doesn't look
good (depending on the theme) so let's prefer ForkAwesome icons within the
tray widget which already uses ForkAwesome in some places like certain
buttons and models. This makes it also look more in-line with Syncthing's
web UI. For context menus and dialogs let's prefer system icons because
there are hardly any ForkAwesome icons used/required so far and it is maybe
nicer to be in-line with the system here.
So far the backend libraries' include paths were relative within this
repository. This means the header files could not be used at their
installed location.
This change replaces them with "<>" includes to fix that problem and adds
a new include directory so building everything at once still works.
With this change it should be easier to actually split some parts into
another repository if this one would become too big.
Martchus