Fix i3-dmenu-desktop quoted command name

Fix i3-dmenu-desktop quoted command name

According to the Desktop Entry Specification
https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#exec-variables
the executable name or path of the executable may be quoted. This is not
properly respected when i3-dmenu-desktop extracts the command name from
the Exec entry.

Examples of values that fail and what they currently result in:

- "bar" -> "bar"
- "foo/bar" -> bar"
- "foo foobar/bar" -> "foo
- "foo\sbar" -> "foo\sbar"
- foo\sbar -> foo\sbar
- "foo\\\\bar" -> "foo\\\\bar"