Issue 18: Added a popup to modify table columns (#31)

Added a new popup to modify the columns of the table. With this new popup, the user can:

- Show and hide columns
- Move columns around
- Add new columns which are derived from the value of an expression
- Delete columns

Also got the overlay mechanisms working.

internal/common/ui/events/commands.go

@@ -16,6 +16,12 @@ func SetStatus(msg string) tea.Cmd {
 	}
 }
 
+func SetTeaMessage(event tea.Msg) tea.Cmd {
+	return func() tea.Msg {
+		return event
+	}
+}
+
 func PromptForInput(prompt string, onDone func(value string) tea.Msg) tea.Msg {
 	return PromptForInputMsg{
 		Prompt: prompt,