optimize filtering

[milianw]

The current filtering is not as efficient as it could be. Most notably, we concatenate the symbol+binary strings. Instead we should:

• inherit from QSortFilterProxyModel
• in its filterAcceptsRow overload, ask the model directly whether it matches the given needle
• in the model, check against the symbol contents

Note that we cannot simply return a Data::Symbol variant, as that would still allocate the data on the heap for type erasure.

[GitMensch]

Is the "concatenate symbol + binary strings" still true?

[milianw]

yes, see e.g. treemodel.h:

        if (role == FilterRole) {
            // TODO: optimize
            return QString(Util::formatSymbol(item->symbol, false) + item->symbol.binary);

[GitMensch]

Then #292 seems to be related performance wise (at least for the "filter out (only this) binary").