Fix for Flatpak apps and add Tym terminal lookup

[PassiveLemon]

Before the get_executable would only return the string "flatpak" from the Exec line while the new function returns the whole string.

I also added Tym terminal to the lookup.

I know #198 fixes this but that PR has been in works for ages now.

[PassiveLemon]

Um nevermind there is a slight problem. It turns out that desktop files which have percent sign variables (like `pcmanfm %U) do not work. They only "worked" before because the sign was never actually included in the executable string as a result of the get_executable function.

[PassiveLemon]

I've got some janky "solutions", each with pros and cons.

1. We keep the string up until the first percent sign: (This allows us to keep any extra arguments in the Exec but may fail due to malformed inputs when the string is cut)

local executable = app:get_string("Exec")
executable = executable:match("^(.-)%%") or executable

2. If there is a percent sign, revert to the old function: (We still can retain original functionality in a bad case but we also keep the downsides from that, aka, the entire reason to try to fix this in the first place)

local executable = app:get_string("Exec")
if executable:match("%%") then
  executable = app_info.get_executable(app)
end

3. Could try to combine 1 and 2 in some way. If the first option fails then we can fall back to original behavior.

[PassiveLemon]

I opted for the first option for now