Numeric stepper issues

https://github.com/HaxeFlixel/flixel-demos/pull/168#issuecomment-93761386

Seems to kind of work now, however there are some things that seem a little counter-intuitve:

• the 0 can't be deleted, when trying to do so, the caret moves to the left though. Trying to enter a dot now doesn't work (should .1 be supported, to mean 0.1 like in most programming langauges?)
• entering 0.6 results in 0.6, entering another 6 in 0.7, another one in 0.8 and another one in 1 - I'm thinking you shouldn't be able to enter anything at all after 0.6 (without deleting a character)?
• typing invalid characters (letters for example) moves the caret around
• shouldn't the + button be disabled when the input value == max value (same for the min value and the - button)?
• negative values seem to be supported, but you can't enter a minus (probably same issue as with the dot?)
• the minus and 0 keys conflict with flixel's sound tray controls (we might need a flag in SoundFrontEnd to disable this temporarily when the steppre has focus)

Start from the numeric stepper implementation and the pull request comment linked in the issue. Reproduce the listed editing, boundary-button, invalid-input, negative-value, and sound-tray conflicts, then verify that the stepper handles the described cases without interfering with sound controls.