The close 'x' was ending up off the right hand side which made
everything else overlap with the left panel, depending on how wide
your window was.
This isn't the end of the dialog problems but it will at least stop
settings from being mangled on develop.
Little bit of a mix of things in this one:
* Support variable-width dialogs. Default is fixed-width as before,
only UploadConformDialog is variable-width. Controlled by a prop
to BaseDialog.
* Fixes to the cancel 'x' - scale the mask image, tweak size & colour
* Colour & boldness of dialog titles
* Align the dialog title & cancel 'x'
* Remove gap between dialog buttons & right hand side of dialog(!)
* Round corners on dialogs
* Add grey border on image preview in upload confirm dialog
* and, squeezing in slightly randomly, finish the partially renamed
ChatInviteDialog to AddressPickerDialog.
