[ticket/13005] Add event to display_forums() to modify category template data

While there're events to modify/add FORUM_POST/FORUM_LINK template data,
such an event is missing for FORUM_CAT forum type.

PHPBB3-13005
[ticket/13006] Add variables to the 'core.modify_quickmod_actions' event

Add $quickmod and $action variables
for the 'core.modify_quickmod_actions' event trigerring
to avoid of using global operator in corresponding extensions.

PHPBB3-13006
[ticket/12976] Correctly use plurals for attachment count in UCP navigation

PHPBB3-12976
[ticket/13042] Remove unused variables

PHPBB3-13042
Merge pull request #2936 from nickvergessen/ticket/12976

[ticket/12976] Correctly use plurals for attachment count in UCP navigation

* nickvergessen/ticket/12976:
  [ticket/12976] Correctly use plurals for attachment count in UCP navigation
Merge pull request #2940 from Senky/ticket/13042

[ticket/13042] Unused var in login_box()

* Senky/ticket/13042:
  [ticket/13042] Remove unused variables
Merge pull request #2903 from rxu/ticket/13006

[ticket/13006] Add variables to the 'core.modify_quickmod_actions' event

* rxu/ticket/13006:
  [ticket/13006] Add variables to the 'core.modify_quickmod_actions' event
Merge pull request #2901 from rxu/ticket/13005

[ticket/13005] Add event to display_forums() to modify category template data

* rxu/ticket/13005:
  [ticket/13005] Add event to display_forums() to modify category template data