Peter Tyser authored Apr 11, 2012 and Kai Willadsen committed Apr 28, 2012

Currently if a user opens multiple file diffs at the same time from a
DirDiff or VcView the following happens:
- the 1st diff is opened in a new tab and focus is moved to this tab
- the 2nd diff is opened in a new tab and focus is moved to this tab
- the 3rd diff is opened in a new tab and focus is moved to this tab
- etc...

This behavior has 2 shortcomings:
1. The user is presented with a window that cycles focus through
   multiple diff tabs which is visually jarring.
2. During this period of time when tabs are being cycled through the
   user can't interact with Meld - the focus shifts too fast to allow
   the user to do anything.  This is especially painful if the user
   initiates a large number of diffs, the diffs are on a slow filesystem
   (eg over a slow NFS share), or if the diffs are of version-controlled
   files (VC-diffs have more overhead, which slows them down).

To resolve these issues, only transfer focus to a new tab if the current
tab is a DirDiff or VcView.  This results in the following behavior:
- the 1st diff is opened in a new tab and focus is moved to this tab
- the 2nd diff is opened in a new background tab
- the 3rd diff is opened in a new background tab
- etc...

While the background tabs are being opened Meld is still usable.

Signed-off-by: Peter Tyser <ptyser@gmail.com>