Always show the scrollbar when usign datetime

This prevents a nasty effect where opening the calendar would
make the scrollbar appear when it was not needed before.

This makes the experience less enjoyable since the glitch always
distracts the user's eyes.

This also as no effect whats so ever on OS X.
Remove un-necessary css

This change remove a float, some padding and all
sizing that is now done by the collapsible.
Use a more precise selector to prevent bleed

This change introduce a more qualified css selector in order
to listen and interact with the collapsible and duplicator plugin.

This reduces then number of possible targets while also preventing
wrong interactions with the plugin since the calendar interface
also contains li elements.
Compatibility with updatesize.collapsible event

This commit need PR https://github.com/symphonycms/symphony-2/pull/2121
in order to work.

This change adds to things: a way to tell the collapsible plugin to
refresh its value cache for height of the content.

It also add a basic prevention in order to optimize things a bit.