Patch to add Keywords to josm{,-latest}.desktop + validation fixes

[sebastic]

The lintian QA tool reports a ​desktop-entry-lacks-keywords-entry issue when desktop entries don't contain the Keywords key or when it does not contain any keywords not already present in the "Name" or "GenericName" entries as documented in the lintian tag description.

The attached patch adds the Keywords entry for both tested & latest, and also fixes the following issues reported by the desktop-file-validate utility:

linux/tested/usr/share/applications/josm.desktop: error: (will be fatal in the future): value "josm.png" for key "Icon" in group "Desktop Entry" is an icon name with an extension, but there should be no extension as described in the Icon Theme Specification if the value is not an absolute path
linux/tested/usr/share/applications/josm.desktop: error: value "application/x-osm+xml;application/x-gpx+xml" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
linux/tested/usr/share/applications/josm.desktop: error: value "Education;Geoscience;Maps" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character
linux/tested/usr/share/applications/josm.desktop: error: value "OpenStreetMap;OSM" for locale string list key "Keywords" in group "Desktop Entry" does not have a semicolon (';') as trailing character

linux/latest/usr/share/applications/josm-latest.desktop: error: (will be fatal in the future): value "josm-latest.png" for key "Icon" in group "Desktop Entry" is an icon name with an extension, but there should be no extension as described in the Icon Theme Specification if the value is not an absolute path
linux/latest/usr/share/applications/josm-latest.desktop: error: value "application/x-osm+xml;application/x-gpx+xml" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
linux/latest/usr/share/applications/josm-latest.desktop: error: value "Education;Geoscience;Maps" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character
linux/latest/usr/share/applications/josm-latest.desktop: error: value "OpenStreetMap;OSM" for locale string list key "Keywords" in group "Desktop Entry" does not have a semicolon (';') as trailing character