online-accounts: always keep editor dialog size the minimum
When adding and editing accounts, the different options that Online
Accounts provides may require different sizes of the dialog. There
is, however, a specific issue where the dialog can't resize because
a provider widget is interefering with the others.
Fix that by making the dialog's stack non-homogeneous, and making
sure to always reset the dialog size before showing it.
online-accounts: Align the panel widgets in the middle
The current implementation of the Online Accounts panel allows
2 states: either the widgets of the panel fill the whole horizontal
space, or they shrink and fill only the absolutely minimum. The
ideal solution, however, is to make them grow with the panel.
Fix that by turn the main box into a GtkGrid, and adding stub widgets
that expand horizontally and pull the main widgets to the middle,
allowing them to cover at most 1/3 of the screen. The widgets themselves
are inside a GtkBox, so that hiding them automatically removes the
spacing in between.
When offline, we currently block the new accounts listbox
so that users can't add new accounts. There is, however,
no visual indication that we're offline, besides the listbox.
Fix that by adding a descriptive label that's only visible
when offline.
