[5.0][Events] Installer event classes

[Fedik]

Summary of Changes

Implementing Event classes for Installer events.

Affected events:

• onInstallerAddInstallationTab
• onInstallerBeforeInstallation
• onInstallerBeforeInstaller
• onInstallerAfterInstaller
• onInstallerBeforePackageDownload

Testing Instructions

Try install/update something

Actual result BEFORE applying this Pull Request

Works

Expected result AFTER applying this Pull Request

Works

Link to documentations

Please select:

• Documentation link for docs.joomla.org: https://docs.joomla.org/Plugin/Events#Installer_Component
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: New Event classes Manual#177
• No documentation changes for manual.joomla.org needed

[HLeithner]

Merging this as with code review and based on the other similar PRs, before beta1 to get enough feedback for final release.

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