Headerbar improvements

[leolost2605]

• Make HeaderBar a separate class that handles the navigation button and therefore reduces duplicate code especially when we want to add more by default (e.g. a menu)
• It also handles switching the maximize button for a unfullscreen one when fullscreened (GNOME Videos does the same). That's because the maximize button doesn't visibly do anything when fullscreened and IMHO only confuses the user. I personally like having a unfullscreen button but another possibility would be to just hide the maximize button.
• Also fixes an issue where it would be impossible to get out of fullscreen on any other page except the PlayerPage

[zeebok]

You have a conflict with master to resolve! :)

[zeebok]

So if my testing shows anything, this alters what happens in fullscreen so that it shows the header and hides it away instead of the unique maximize button it used to have, correct? It certainly works better! I noticed the maximize button state can get confused in the current version.

[leolost2605]

So if my testing shows anything, this alters what happens in fullscreen so that it shows the header and hides it away instead of the unique maximize button it used to have, correct?

I think what you mean was already implemented in #361 :)

I noticed the maximize button state can get confused in the current version.

With current version you mean main? If not then there is something wrong here

[zeebok]

With current version you mean main? If not then there is something wrong here

Sorry, yes, I mean the current release on AppCenter. Your version seems good!