Host programms not runnable from flatpak darktable

Main reason for me to use lua scripts is to be able to
* open image in gimp
* Create HDR Image
* Reveal in file browser

However, when trying any of these lua scripts, they are blocked by not able to find the associated executable
in the flatpak sandbox, however the host apps are installed and work flawlessly.

Recently i came across a solution based on

https://github.com/flathub/org.gnu.emacs/issues/14

where the app can be started by flatpak-run.
This now works for me for HDRMerge, since i modified the file HDRMerge.lua like this:

```
local function BuildExecuteCmd(prog_table) --creates a program command using elements of the passed in program table
local result = 'flatpak-spawn --host '..CleanSpaces(prog_table.bin)..' '..CleanSpaces(prog_table.arg_string)..' '..CleanSpaces(prog_table.images_string)
return result
end

```
and added the dbus permission using flatseal to darktable.

This works perfectly, but needs to be done for every script which calls a host executable.

However it would be impractical to modify all scripts to also work in flatpak also.

### Proposal
Extend function
`'dtutils.system.external_command()`
to prepend the string "'flatpak-spawn --host "
to the exec command when running in flatpak environments.

Or add a new command "external_host_command()" which has these semantics, when the former should not be modified....

Start with dtutils.system.external_command() and compare its behavior with the modified HDRMerge.lua example. Check how darktable detects Flatpak environments and how the required host-spawn permission is represented. Done means host executables can be invoked from Lua scripts without modifying each script, while existing non-Flatpak behavior remains intact.

Written by the indexing model from the issue text.