Deferred edittoolbar tooltip init to when switching to edit mode

Change-Id: I46772ea8cbb9457654070b2405c2b30428e37c53
Remove duplicate getContentForRequest call in ViewEntityAction

There is no good reason to call it again in the isPlainView method,
just to check if it is instanceof EntityContent, since
ContentRetreiver::getContentForRequest is already called previously
in the show() method.  There, it is checked if null, then displays
missing entity, else is passed to displayEntityContent(),
with type hint. Then, isPlainView is called in displayEntityContent,
and at athat point we already know we have EntityContent. No need
to check again.

ViewEntityAction::isPlainView is called nowhere else in Wikibase
or in core, so also can be made private, and also give it a more
appropriate name, isEditable().

Bug: 72158
Change-Id: I7453e1f72f12d6dbf14d029451a4a9a8d360a4e0
Merge "Remove duplicate getContentForRequest call in ViewEntityAction"
Merge "Deferred edittoolbar tooltip init to when switching to edit mode"