Refactor toolbar button code into separate button classes

[martinpovolny]

Toolbar button special behaviors have to be handled by separate classes under app/helpers/application_helper/button.

Turn branches for specific button IDs in build_toolbar_hide_button and similar functions to such classes.

E.g. button changes from #5884 should be implemented in a separate class representing the particular buttons.

ping @romanblanco

[martinpovolny]

See #5959 #6078 #5783

[chessbyte]

@martinpovolny I see a lot of merged PRs for this issue. Is there a way to track how many remain to be converted and what completion percentage is?

[romanblanco]

@chessbyte AFAIK, @vecerek has already finished or maybe missing last PR with toolbar buttons conversion

[vecerek]

@chessbyte the last 3 toolbar buttons remain to be converted

[romanblanco]

@martinpovolny I believe you can close this issue now 😉