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.
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.
user-accounts: Fix crash when name is changed and dialogue closed
Wrong variables are used to store timeout id, consequently, the sources
are not properly removed in all cases. Then the dialog may crash in certain
cases if it is closed before the source functions are called.
