CellML Text view: improve Unicode support

[agarny]

Whenever the position of the editor's caret changes (e.g. when editing a CellML file), we extract the statement that corresponds to an equation, so we can render it. However, there are cases where we might be editing a comment that is just before an equation, and which will return the wrong statement (because of the way the comment is being edited):

There are also cases where things will go wrong if we have certain Unicode characters. For example, say that we have the following CellML file:

def model my_model as
    def unit ms as
        unit second {pref: milli};
    enddef;

    // フランス人

    def unit mV as
        unit volt {pref: milli};
    enddef;

    def unit mV_per_ms as
        unit mV {expo: 1};
        unit ms {expo: -1};
    enddef;

    def comp main as
        var time: ms;
        var v: mV {init: 1};

        // dv/dt = 1
        // v(0) = 1

        ode(v, time) = 1{mV_per_ms};
    enddef;
enddef;

フランス人 will render our update viewer completely wrong (because of those characters taking 3 bytes rather than 1). Then, we also have to be careful about some Unicode characters being used in our partial statement. For example, if we were to copy/paste フランス人 in the dv/dt comment, then this would mess up or check in CellmlTextViewWidget::statement().