feat: add priority to connect, to control callback order

[tlambert03]

This adds the ability to control the order in which callbacks are called at connection time. Higher priority connections are called first, and negative priorities are allowed. with default 0

sig = SignalInstance()

calls = []

sig.connect(lambda: calls.append(1))
sig.connect(lambda: calls.append(2), priority=5)
sig.connect(lambda: calls.append(3), priority=-5)
sig.connect(lambda: calls.append(4), priority=10)

sig.emit()
assert calls == [4, 2, 1, 3]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (63573f3) to head (8696e69).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #285 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 1919 1931 +12 ========================================= + Hits 1919 1931 +12 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #285 will not alter performance

_{Comparing tlambert03:priority (8696e69) with main (63573f3)}

Summary

✅ 66 untouched benchmarks

[tlambert03]

@Czaki, let me know if you have any feelings about this feature or its implementation. This is similar to position in vispy's events... but without the whole before/after positioning functions. The connector simply gives a priority number by which to order the callbacks; with zero being the default, and higher priority callbacks getting placed before lower priority ones.