Improve Java-to-Kotlin converter

Although the J2K-converter already supports a range of Java constructs, there are still some missing PSI node types that currently cannot be converted. These include, but are not limited to:

* Anonymous classes
* Switch statements
* Synchronized statements
* Try-catch
* Throw statements
* Annotations (especially `@Override` -> `override`)

Implementing them is fairly straightforward, since they directly correspond to a method in [`JavaElementConverter`](https://github.com/fwcd/KotlinLanguageServer/blob/master/server/src/main/kotlin/org/javacs/kt/j2k/JavaElementConverter.kt):

https://github.com/fwcd/KotlinLanguageServer/blob/93e633951c089d713d7a8c07f18ea389cfa690fa/server/src/main/kotlin/org/javacs/kt/j2k/JavaElementConverter.kt#L76-L620

Additionally, there are some improvements to be made to the existing conversion methods:

* Supporting all [modifiers](https://www.w3schools.com/java/java_modifiers.asp) (such as `private`, `protected`, `transient`, `abstract`, ... - `static` and `final` currently the only supported ones)
* Note that these modifiers can be found in [PsiModifier](https://github.com/JetBrains/intellij-community/blob/master/java/java-psi-api/src/com/intellij/psi/PsiModifier.java)
* Automatically converting getters/setters to the "Kotlin style"
* `getSomething()`/`setSomething(...)`-style methods can be converted [using the same rules as the completions](https://github.com/fwcd/KotlinLanguageServer/blob/93e633951c089d713d7a8c07f18ea389cfa690fa/server/src/main/kotlin/org/javacs/kt/completion/Completions.kt#L120-L125)
* Some conversions, specifially those for which Kotlin maps a "platform type to a Java class" are tricky, since they follow no special rule
* Consider `java.util.Collection.size()` -> `(kotlin.collections.)Collection.size` (which is a property)
* Automatically converting classes with no fields to `object`s

This is not an exhaustive list and other improvements are welcomed.

Start with server/src/main/kotlin/org/javacs/kt/j2k/JavaElementConverter.kt and inspect the existing conversion methods for the listed PSI nodes and modifiers. Compare getter/setter conversion rules in server/src/main/kotlin/org/javacs/kt/completion/Completions.kt, then verify that the requested Java constructs, modifiers, accessors, and fieldless classes convert correctly.

Written by the indexing model from the issue text.