[RFC] Make the "LaTeX Equation" dialog modal

Hi all,

The current "LaTeX Equation" dialog, as well as the "Find", "Replace"
dialogs, is a modeless one. As a result, the user can work on the
document in the background when it is opened up.

However, the "LaTeX Equation" dialog is somehow different, in that it
operates on a specified object at a specified place, just like the
"Format"->"Font" dialog. But the dialog has no idea which object
(equation) is being edited. If the user points the mouse at some other
place in the document before clicking the "Insert" button, then
another equation is created/modified instead the original one, which
is very confusing.

Therefore, I suggest to make this dialog model. Any thoughts or comments?

//I have discussed this on IRC with sum, who suggested me to bring it here :)

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Thanks & regards
Xun Sun