GPS plugin - use stored path for all file browser dialogs; also save/use file type

[smizuno]

1. GPS Tools (GPS Importer) - use stored path to GPX file to initialize the file dialog (and save on successful operation) in the following tools/file browsing:

Import other file - "GPX output" field

Download from GPS - "Output file" field

GPX conversions - "GPX output file" field

as is done for Load GPX file (File) and GPX conversions (GPX input file)

Also do this for Create new GPX layer.

I reason that most users will probably be storing GPX files in the same general location, perhaps one directory up/down from there, plus it is very annoying to have to navigate from current working directory every time the file browse dialog is used.

2. use a different stored path to initialize the file dialog (and save on successful operation) in the Import other file - "File to import" field

Same reasons as in item 1, except that this is a different type of file that may be stored in a different location.

3. store the type of file and initialize the file dialog from this for "File to import" in the Import other file tool.

Reason is that a user is likely to be importing a number of files of the same type, so it is annoying to have to set the type each time.