Enforce minimum and maximum map scale limits

[tomasMizera]

Problem

The map currently allows zooming out and in without any bound, which causes several problems:

• Zooming out too far breaks the scale bar display (see Zooming out too much doesn't corespont with the scale #1557 — scale bar starts showing incorrect/wrong length once zoomed out past a certain point, e.g. ~1000km).
• There is no benefit to zooming out further than having the whole world map fit the screen — it's just wasted zoom range and a source of bugs.
• Zooming out too far also interacts badly with geometry editing handles (see Geometry editing handles visual problems #2280) — handles/vertices become impossible to work with at extreme zoom levels, and recording guidelines can stretch incorrectly across the screen.

From #2280 (comment):

In addition to hiding the handles when geom is too small, we should introduce and enforce a minimum map scale. There is no point in zooming out further than having the world map fit the screen!

Proposal

• Introduce and enforce a minimum map scale — the map should not zoom out further than the point where the world map fits the screen.
• Introduce and enforce a maximum map scale — cap how far in the user can zoom, to a sensible upper bound.
• These limits should apply generally to map navigation (pan/zoom), not just during geometry editing.

Related

• Zooming out too much doesn't corespont with the scale #1557 — scale bar shows incorrect values when zoomed out too far; also requests a zoom limit
• Geometry editing handles visual problems #2280 — geometry editing handles visual problems; comment above originates the min-scale idea in this context