Use supported extensions as filters in "Open File" dialog.
Add Vim swap files to gitignore.
Encapsulate "Add..." buttons in a class to avoid inconsistencies.

"Add..." buttons are created in two different locations in the code. To
avoid inconsistencies in text or options, this patch moves all
initialization to a subclass of QPushButton.
Remove "Add Many by Pattern" option to use a glob pattern to add files.

A similar feature may be implemented based on the file selection dialog
for stacks. The current UI will not be useful in the future.
Move "Add..." button to a row of the datasetDetail view.

In the data selection applet, the summary view includes the "Add..."
button on the last row of the table. This patch changes the rest of the
views to do the same.

Extra row logic used for the summary view is refactored into a new class
and used again for the datasetDetailedInfoTableModel.
Trivial fix for signal signature in AddFileButton.
Remove "Remove Lane" button from data selection summary view.
Data selection, raw data table: show remove button on row under the cursor.
DataSelection: Show add button in each row without data.

"Add..." button behaviour in the current data selection applet is to
fill the first empty lane for the given role with the specified file.
This can lead to confusion when displaying the add button on the last
row of the table.

This patch displays and "Add..." button on each empty row. The main
"Add..." button on the last row is preserved and renewed to "Add New..."
DatasetDetailedInfoTableView: Remove "Add File" buttons when they aren't needed anymore instead of just hiding them.
(Otherwise they can steal events from the parent view, even if they are invisible.)
AppletSerializer: Don't import volumina until it is needed (support headless mode).
Merge pull request #761 from burcin/data_selection

Revamp data selection applet GUI.
DataSelection: Force a repaint of the table after the user scrolls.  (Fixes a Mac-only issue.)
Merge branch 'data_selection_guifixes'
DataSelection: Preliminary UI support for selecting and loading a DVID volume (doesn't save to a project file...)
DataSelection: Fixes to handle urls.
DataSelection: Add dvid volumes as urls, not temp files.
DataSelection: Support url edgecase.
DataSelection: dvid: Hostname is now provided by the contents browser ui.
Applet GUIs fixes:
- In stopAndCleanup(), don't touch QWidgets that might have been deleted already.
- When handling a ThunkEvent, check to make sure the object is still alive.