toolbar selects and visibility improvements

[kevinpapst]

Description

• Unify behavior of customer/project/activity dropdowns
• build optgroups for API results in dropdowns
• allow to reset all fields
• fix visibility filter in customer/project and activity administration screens

Fixes #817
Fixes #820

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I verified that my code applies to the guidelines (composer kimai:code-check)
• I agree that this code is used in Kimai and will be published under the MIT license

[lock]

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