symbols: Added support for *multiple actions per levels:* - *1 keysym == 1 action* now holds for every keysym count. - When no action is specified, `interpret` statements are used to find an action corresponding for *each* keysym, as expected. - When both keysyms and actions are specified, they should have the exact same count for each level. - For now, at most one action of each following categories is allowed per level: - modifier actions: `SetMods`, `LatchMods`, `LockMods`; - group actions: `SetGroup`, `LatchGroup`, `LockGroup`. Some examples: - `SetMods` + `SetGroup`: ok - `SetMods` + `SetMods`: error - `SetMods` + `LockMods`: error - `SetMods` + `LockGroup`: ok