Add the 'Import files' functionality

Similar to File->Open but for multiple files and without
calling on_actionClose_triggered().

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add action to 'File->New'

File->New in the GTK version performed the same as File->Close.
Creating a new file may also ask the user if the current file has
to be saved.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Enable 'View->View Globe'

For viewing the Globe section (pane) we need to collapse
both the 'list' and the 'main' splitters.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Reorder the 'View->View ..' list

Use Ctrl+1 for 'View All', while all the singular views follow
with Ctrl+2..5.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Update main window title depending of current file state

If a file has been opened from the command line or via the File
menu the main window title becomes "Subsurface: filename.ext".
Title also updates if 'File->Save As' is called. "Subsurface" only
is displayed when no active file is present or post 'File->New'
or 'File->Close'.

To make this work a new public method is added - MainWindow::setTitle()
and also an enum type MainWindowTitleFormat, which should allow
more complicated formatting, such as showing the selected dives
or the total number of dives (e.g. MWTF_FILENAME_N_DIVES).

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>