Localization Support for Zoom Control

[Danielku15]

I added localization support for the zoom control. There was a pull request for this before: #1643 But the implementation was quite strange with language overriding.

This implementation uses the control options to localize the strings. I also thought of another solution with:

options: {
    buttons: {
        zoomIn: {
            title: 'Zoom in',
            text: '+'
        },
        zoomOut: {
            title: 'Zoom out',
            text: '-'
        }
    }
}

Let me know what you'd prefer and I will implement it.

[mourner]

Sorry for a later reply, just got back from vacation. Thanks for the pull!
Is there a specific need to localize the +/-? They seem language-independent.

[Danielku15]

I made the +/- localizable to allow people the use of custom unicode symbols as zoom indicators. Or if you want to use a custom web-font you can override the characters used for the zoom buttons.

[mourner]

OK, makes sense! Can you rename zoomIn/OutTooltip to zoomIn/OutTitle for better naming consistency? I'll merge then.