UI improvements

[Mistress-Anna]

In admin mode, when clicking an existing item in the center panel, an "edit box" is shown. The input field from the box should be focused automatically (or the first input if there are several).

After adding a "single line text box", I click on it to edit its label, the edit box appears with 2 input fields. I click on the first input which shows "Label". The cursor is automatically focused before the first character and I can't select the word "Label" anyway, like a placeholder in the background.
However, when I type a letter like "x", the input contains "xLabel", so the "x" didn't replace the placeholder text, this is weird. HTML's tag can have a replaceable "placeholder" attribute.

Also, it's not clear the user should press "enter" to validate her editing in that "edit box". If she clicks on another fields, the modifications are lost, and there's not "validate" or "modify" button, so only "enter" can be used. Either add a button in the edit box, or validate automatically (and update center panel) on each character typed.

[almet]

Have you tested this? #20 I should actually merge this PR :-)

[Mistress-Anna]

Sorry, I'm not really a web developer, so I couldn't really fix the conflicts git made on master.