No way to exit a TakeOff command from the context menu

#### Issue details
This is a minor UX issue that can cause a more dramatic issue when coupled with an apparent bug in ArduPlane (https://github.com/ArduPilot/ardupilot/issues/12035)

1) When you select TakeOff from the right-click context menu, the altitude modal that appears is identical to the Guided Alt one, which means you can think you're setting a guided mode altitude when you're actually issuing a TakeOff command.

2) There appears to be no way to stop the TakeOff happening once you select it from the menu, by that I mean pressing "Cancel" or "X" on the altitude modal results in a TakeOff instruction happening anyway - I'm not sure if this is intentional behavior or not.

#### Version
1.3.68.1 build 1.3.7160.16169

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X ] Plane
[ ] Rover
[ ] Sub

Start by reproducing the Plane TakeOff action from the right-click context menu and trace the altitude modal shown afterward. Confirm that Cancel or X does not issue TakeOff, and that the modal clearly distinguishes TakeOff from Guided Altitude before considering the issue done.

Written by the indexing model from the issue text.